<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            color: #428BCA;
            margin: 0;
            padding: 0;
        }


        h1 {
            padding: 10px 20px;
            text-transform: uppercase;
        }

        h2 {
            padding: 0 20px;
        }

        .nav {
            height: 49px;
            width: 100%;
            border-bottom: 1px solid rgba(128, 128, 128, 0.6);
            z-index: -1;
        }

        .nav a {
            box-sizing: border-box;
            color: #428BCA;
            text-decoration: none;
            float: left;
            display: block;
            height: 50px;
            margin: 0 auto;
            padding: 18px 30px;
            font-size: 14px;

        }

        .nav a:focus {
            border-left: 1px solid grey;
            border-right: 1px solid grey;
            border-top: 1px solid grey;
            border-top-left-radius: 5px;
            border-top-right-radius: 5px;
            border-bottom: 1px solid white;
            color: #555555;
        }
    </style>
</head>

<body>
    <div class="nav">
        <a href="#Tab1">TAB1</a>
        <a href="#Tab2">TAB2</a>
        <a href="#Tab3">TAB3</a>
    </div>
    <main>
        <h1>this is TAB1 content</h1>
        <h2>Notice the gap between the content and tab after applying a abckground color</h2>
    </main>
</body>

</html>